首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6134篇
  免费   576篇
  国内免费   922篇
  2024年   31篇
  2023年   172篇
  2022年   236篇
  2021年   276篇
  2020年   286篇
  2019年   296篇
  2018年   281篇
  2017年   325篇
  2016年   342篇
  2015年   316篇
  2014年   326篇
  2013年   425篇
  2012年   279篇
  2011年   284篇
  2010年   255篇
  2009年   311篇
  2008年   309篇
  2007年   362篇
  2006年   298篇
  2005年   277篇
  2004年   212篇
  2003年   208篇
  2002年   177篇
  2001年   149篇
  2000年   154篇
  1999年   135篇
  1998年   103篇
  1997年   84篇
  1996年   79篇
  1995年   95篇
  1994年   67篇
  1993年   86篇
  1992年   69篇
  1991年   49篇
  1990年   47篇
  1989年   43篇
  1988年   28篇
  1987年   36篇
  1986年   16篇
  1985年   21篇
  1984年   13篇
  1983年   12篇
  1982年   27篇
  1981年   13篇
  1980年   7篇
  1979年   5篇
  1978年   3篇
  1977年   2篇
  1976年   3篇
  1973年   2篇
排序方式: 共有7632条查询结果,搜索用时 78 毫秒
61.
Inorganic nitrogen processing and assimilation in a forested wetland   总被引:1,自引:1,他引:0  
Feeding selectivity by five epilithic, case-building trichopteran species from a first-order stream was examined using laboratory experiments. Larvae had the choice of three food types: periphyton and detritus from the stream, and nettle broth as an artificial food source. Food type influenced the microdistribution of these species. Larvae were able to distinguish between different food types and showed species-specific responses to the various types of food. Drusus annulatus, Micrasema longulum and Apatania fimbriata favoured periphyton, while Agapetus fuscipes and Silo pallipes showed no preference for periphyton over detritus. Four of the five species examined (Apatania fimbriata was the exception) tended to avoid nettle broth. The complexity of foraging behaviour was illustrated by M. longulum. Given a choice between detritus, periphyton and periphyton with the addition of moss, they clearly preferred the latter food type, scraping mostly on the epiphytic algae. Proportions of a given species selecting periphyton were correlated with percentage areas of algae in the gut contents of that species in the field. D. annulatus, M. longulum and A. fimbriata showed the greatest preference for periphyton, and consumed the highest proportions of algae in the field. A. fuscipes and S. pallipes often selected detritus, and these species had the lowest proportions of algae, and the highest proportions of detritus, in their guts.  相似文献   
62.
After considering the need for quality control in NAA, the concept of quality in NAA procedures themselves is discussed, and some important factors identified. Two approaches to improve quality are then described in more detail. The first concerns the unique ability of NAA using different isotopic reactions and different modes (INAA/RNAA) to provide independent data sets in the same laboratory, thus allowing internal validation or crosschecking. The second discusses the need for chemical yield measurements in RNAA and the advantages of the radioisotopic tracer technique. Some recent advances and further possibilities for this use of tracers are listed.  相似文献   
63.
A group of Colobus polykomosat Tiwai, Sierra Leone, demonstrated seasonal flexibility in its diet, with seeds, young leaves, and mature leaves each dominating the diet at different times. Comparison of food consumption with phenological data indicates that seeds are eaten whenever available and are preferred to other foods, while young leaves are preferred to mature leaves. Colobus polykomosalso prefer liane to tree leaves, despite the relatively high quality of mature tree foliage at the Tiwai site. Analysis of protein, fiber, and energy values of foods selected and items available, but not eaten, suggests that preference is related to protein and energy maximization. Leguminous plants, especially Papilionaceae and Mimosaceae, are highlighted as important food sources for C. polykomos;seeds and leaves from these families have a high nitrogen content, and the protein content of leguminous seeds often equals or exceeds that found in leaves. It is predicted, therefore, that colobines living in habitats with a high density of legumes will feed heavily on seeds, subject to constraints such as seasonal availability. Where suitable leguminous species are less common, a mixture of fruits, seeds, and young or mature leaves or both is likely to be selected. The results of this and other recent studies of colobines do not support the notion that colobines are specialist folivores.  相似文献   
64.
The induction of parthenocarpic fruit set was investigated using the apple cvs. Golden Delicious and Jonagold. The gibberellins GA3, GA4, GA5 and GA7 and the synthetic phenylurea-type cytokinin CPPU (N-(2-chloro-4-pyridyl)-N-phenylurea), were applied alone and in combination to unpollinated flowers at the end of petal fall. Gibberellins induced only a marginal final set of parthenocarpic fruits. CPPU sprays were more effective, particularly in the first year. When applied in combination, CPPU and gibberellins had a positive synergistic effect on parthenocarpic fruit set and fruit size, but a negative effect on flower induction the next year. After CPPU + GA sprays, percent fruit set was similar, or greater, compared to natural pollinated trees. The parthenocarpic fruits induced by CPPU + GA had an increased length to diameter ratio. CPPU stimulated, and GA4 and GA7 reduced, the russeting of the parthenocarpic fruits. The internal quality of the fruits was hardly affected, but Ca-deficiency symptoms occurred more frequently in parthenocarpic fruits.  相似文献   
65.
Somatic cell and gene therapy involve the application of biological technologies to an individual patient through the use of living cells which provide a therapeutic benefit (Aliski, 1991). Various forms of cellular and gene therapies are being developed and evaluated in an increasing number of clinical trials for congential and acquired disorders. The potential and progress of these therapeutic applications have resulted in an increasing effort by the Food and Drug Administration (FDA) to develop the regulatory framework under which these therapeutic approaches would insure safety and efficacy, the primary mandate of the FDA.Over five years ago Cellcor began to define the parameters, specifications, and conditions relevant to a Quality Assurance/Quality Control (QA/QC) program that has evolved to insure safety and maximize the efficacy of applications of the company'sex vivo technology, autolymphocyte therapy. Autolymphocyte therapy is an outpatient form of somatic cell immunotherapy based upon the infusion of T cells that have been activatedex vivo using a combination of previously generated autologous cytokines and an anti-CD3 monoclonal antibody.We have been able to demonstrate the feasibility for the safe, controlled, and consistent preparation and delivery of a cellular therapy by application of relevant GMP regulations. This presentation reviews aspects of this program and chronicles our experience which at present amounts to over 4400 infusions for over 700 patients. This program provides a high degree of assurance that a cellular therapy program can be carried out in a multisite mode involving hundreds of patients through the strict adherence to cGMP as set forth in existing regulations. It would be prudent that developers of cellular andex vivo gene therapies establish a similar cell processing and QA/QC infrastructure at an early developmental stage to optimize safety and reproducibility and facilitate regulatory review.  相似文献   
66.
生态环境对川芎产、质量的影响   总被引:3,自引:0,他引:3  
生态环境对川芎产、质量的影响丁德蓉,陈兴福,卢进,黄文秀,刘岁荣(四川省中医药研究院药物种植研究所,南川648408)InfluenceofEcoenvironmentontheYieldandQualityofLigusticumchuanxion...  相似文献   
67.
不同配比的GA_3和KT与营养液配施,对平菇菌丝生长影响不同。GA_3和KT的适当配比,能促进菌丝分枝生长,也能显著提高平菇子实体的产量和品质,但维生素C的含量有所下降。GA_3和KT若与营养液配合施用,比单独施用更能发挥其促进作用。  相似文献   
68.
长白山站的研究数据库管理系统翟永华,赵士洞(中国科学院沈阳应用生态研究所110015)Date-BaseManagementSysteminChangbaishanForestEcosystemResearchStation.¥ZhaiYonghua...  相似文献   
69.
The sex ratio in final-instar larvae of a birch-feeding, free-living solitary sawfly, Dineura virididorsata, was investigated in Finnish Lapland. The prepupal proportion of females, pooled over ten sites, was 56%, and at four individual sites the sex ratio was significantly female-biased. Larval survival from egg to prepupae did not differ between the sexes. This suggests a femalebiased primary sex ratio in the field. The sex ratio varied among the sites but not among host trees within sites. Contrary to previous results with hymenopterans, we did not find that differences in the sex ratio depended on forage quality: site-specific or tree-specific sex ratios did not correlate with the average prepupal weight. A literature search indicated that female-biased sex ratios are also common in other free-living sawflies. We are unable to explain sex ratios of Dineura virididorsata or other free-living sawflies with existing general models.  相似文献   
70.
An important component of the interaction between macroinvertebrates and leaf litter in streams in the extent to which consumers can differentiate between undecomposed and decomposing leaves. The detritivores Gammarus pulex and Asellus aquaticus fed preferentially on conditioned rather on unconditioned leaf material. Growth in A. aquaticus was significantly reduced when unconditioned leaves were provided, but in G. pulex no significant effect of conditioning on growth was observed. The capacity of G. pulex to tolerate reductions in food quality seems to be a consequence of a compensatory system in which respiration rates change to compensate for reductions in food quality. In this way a constant growth rate is maintained. Increases in ingestion rates to compensate for low quality food were not observed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号